A topsail is a sail set above the lowest sail on a sailing ship's mast. Picture a tall ship with multiple square sails stacked vertically: the bottom sail catches wind near the deck, and the topsail rises above it to capture even more wind higher up, where the breeze blows stronger and steadier.

On square-rigged ships (the kind you see in pirate movies or old naval paintings), topsails were crucial for speed and power. Sailors had to climb high up into the rigging to adjust these sails, working on narrow ropes and wooden bars called yards that held the sails in place. Managing topsails required skill, courage, and teamwork, especially during storms when captains might order crew to “reef the topsails” (reduce their size) to prevent damage.

The word appears in many sailing phrases and ship names. A topgallant sail sits even higher, above the topsail. Some famous ships were called topsail schooners, meaning they combined traditional triangular sails with square topsails for extra speed. When someone says a ship is running under “full sail,” they mean every sail, including the topsails, is unfurled and pulling.